Selyf, son of King Cynan Garwyn, was
a great warrior king: his nickname was Battle Serpent. He took a
Powysian army north, in 613, to join the united British force that was to
fight King Aethelfirth of Bernicia at the Battle of Caer-Legion (Chester,
Ches.). Aethelfirth had invaded Gwynedd in order to stop King Iago of
Gwynedd restoring the former's old enemy, Edwin, to the Deiran throne.
Unfortunately the British were severely defeated and Selyf and Iago were
both killed.